It is a modified Prusa i3 printer, the whole extruder assembly is completely clean and free of debris. The two wheels are under the correct tension. The plastic does have the small dimples. When the filament does not extrude, it is not being stripped either, the motor simply twitches back and forth, like it’s too weak to do the job.
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Thanks for the reply, I’ve ordered new wiring. I’m not sure it’s the control board, because every other motor works perfectly, and it’s a new board that I had replaced rather recently.
I now know it’s not the hot end, I’ve installed it on another printer and it worked printed fine. I checked the vref, and it seems to be sitting at 0.2. How much would I have to turn the trim pot on the stepper driver to increase this? Or is that just for increasing the current?
Is it a RAMPS board? If so, check to make sure you have all the jumpers in place. Got tripped up by that one more than once. Try swapping out drivers.
A whole dollar? 
when steppers fail, it’s usually due to overheating which can warp the core where the windings are wrapped. This will cause the rotor to “hunt” for its index. In my case, the stepper would oscillate back and forth.
Update (Problem has been located):
- I switched the cables around, and there was no change.
- I switched the stepper drivers for the extruder and the z-axis, and the filament started extruding more smoothly than ever before!
- I had switched the drivers before, but with a different motor for the extruder. With the original motor for the printer, and the z-axis driver, it worked.
Thank you to everyone who helped, hopefully when the new driver gets here, there will be no more issues!
It actually was the drivers! I switched them out the first time, using the newer motor I had, and nothing different happened. When I switched to the motor that came with the printer, and then switched the drivers, it started extruding plastic perfectly. Thanks for the help.
I experienced something very similar, mine was harder to track down as it would only occur occasionally during prints. My printer would start out fine, then some time during the print my extruder would start jerking like you described. I thought it was temperature so I added active cooling, which did not help, I finally tracked it down to the plug on the end on the cable where it plugs into the stepper. The way mine is set up the flexing over time wore it out, and instead of trying to order a new one from china, and possibly having the issue again, I opened up the stepper and hard wired the cable, then built in a little strain relief using the case of the stepper itself. That was probably close to a year ago, and have not had an issue since. But in diagnosing this you need to be careful with the steppers being unplugged while being energized.
2 years later
I am having same issues. Filament wont feed. Stepper motor goes forward and back. Here what I have done. Swamped stepper motor cable - Cable was fine. Replaced complete extruder tip and tube - No difference. Changed Stepper motor - No Difference. Adjusted Stepper motor current - No difference Changed gear on feeder - No Difference. Changed guide wheel on extruder - No difference. Tried stepper with no filament in it or tension on the guide wheel - Motor still doing same thing. Reflash controller - No Difference. Replaced controller board - No difference. Reflashed new controller board - no difference .Checked power supply Voltages - All voltages rate where they should be. Scratched my head - no hair left. The only thing left is software. Same thing is happening in Cura and Mattercontrol. Have also reinstalled both. Help I am going crazy with this.
Does the original stepper have this issue plugged in somewhere else?
This still sounds like a cable issue.